1. Field of the Invention
The present invention relates to an initializing apparatus for initializing a state of a recording layer of a phase change optical disk (hereinafter referred to as an "optical disk").
2. Description of the Related Art
A first prior art device will be described below with reference to FIGS. 5(a) and 5(b).
In the first prior art device, laser light L is emitted from a gain waveguide type high output semiconductor laser 50. The laser light L is focused on an optical disk D through a collimator lens 52 and an objective lens 54. In cases where the objective lens 54 has a NA (numerical aperture) of 0.5, the size of the spot S.sub.10 on the optical disk D is 1.times.50 .mu.m.sup.2 at the half-width of the beam intensity distribution. The initialization of a recording layer of the optical disk is accomplished by crystallization of the whole region in the spot S.sub.10 of the recording layer, irradiated by the erasing laser light from the laser 50 driven by a DC (direct current) signal (referred to as a "DC drive" below) to emit at a constant output.
A second prior art device will be described below with reference to FIGS. 6(a)-6(d).
The second prior art device is disclosed in Japanese Patent Application Laid-open No. Hei 4-278224. Unlike the first prior art device, a pulsed output of the laser light is used for initialization in this case, and a broad area is initialized with the same spot size of the laser light.
FIG. 6(a) shows a condition of the recording layer irradiated with laser light driven by a one-shot drive pulse. A melting amorphous portion S.sub.11 is the region where the recording layer melts during the one-shot pulse laser drive and is rendered amorphous after cooling, and a melting re-crystallization ring S.sub.12 is the region where the recording layer melts during the one-shot pulse laser drive and crystallizes after cooling.
By making the output of the laser stronger than in the case of the first prior art device, a spot periphery S.sub.12 in a crystalline state has a large outer diameter. But the light quantity of the center portion S.sub.11 of the spot exceeds a threshold level for forming an amorphous state and a center portion S.sub.11 of the spot is rendered amorphous. The laser is repeatedly driven by switching by pulse signals having a low duty ratio (referred to as a "pulse drive" below) with the cycle time corresponding to the width of crystallined portions as shown in FIG. 6(c) so that a crystalline portion of the spot periphery S.sub.12 for each pulse is linked with that produced by the next pulse as shown in FIG. 6(b). By repeating this pulse drive, the whole recording layer is crystallized as shown in FIG. 6(d)
However, the above-described prior art has the following disadvantages:
1. When an initialization is performed by driving a laser light source in a DC drive as in the first prior art device, a noise (media noise) is considerably caused due to the recording medium. Thus, a sufficient S/N ratio cannot be obtained, thereby resulting in the deterioration of a bit error rate. PA1 2. When the initialization is performed by a pulse drive as in the second prior art device, the sizes of the melting re-crystallization ring portions vary. This also causes media noise. PA1 3. A track address, which is partially recorded on an optical disk, is also undesirably initialized.